The direct link between satellites for the transmission of information signals was initiated approximately 20 years ago and to this date almost exclusively employs radio frequencies (RF). The expansion of the commercial satellite industry by means of multi-satellite programs such as IRIDIUM (Iridium, Inc.), TELEDESIC (Teledesic Company), SPACE-WAY/GALAXY (Hughes) and others represents a technological evolution of a scope not seen up to now, in particular in connection with mobile radio communications. The placement of several thousand inter-satellite terminals in space is planned in connection with this. Although as a rule the use of 60 GHz RF links is planned, tests employing laser technology have also become known.
In contrast to the technologically mature and commercially wide-spread optical communication by means of glass fibers, the satellite laser communication is still in the research and development phase. The advantages of a laser communication in contrast to RF communication, however, are
a high transmission capacity up to the multi-Gbit range, thanks to the laser light frequency which is greater by approximately four tens, compared with microwaves, PA1 high immunity to interferences, PA1 solution to the RF problem of congestion of the electromagnetic spectrum, PA1 small antennas, PA1 small, compact and light on-board terminals, possibility of miniaturization thanks to integrated and diffractive optical devices, compact laser sources and laser arrays, PA1 low electrical power requirements. PA1 a diode laser for communication and pointing, acquisition and tracking, PA1 an optical system of a two-mirror application for providing a two-stage pointing and alignment device for the purpose of aligning the transmitting and receiving beams, PA1 the combination of hybrid optical components, diffractive elements as well as refractive/reflective components, PA1 a control of the optical alignment by means of coherent superimposed reception, PA1 an optical phase modulation by means of semiconductor amplifiers, PA1 a transmission bit rate from&gt;100 Mbit/s to 1.5 Gbit/s, preferably 1.5 Gbit/s, and a transmission distance from&gt;500 m up to 4,500 km, preferably 1,200 km.
In spite of these advantages, a demonstration of dependably operating on-board terminals has not succeeded so far. Laser transmissions would have the potential of presenting successful solutions to the rapidly increasing requests for transmission capacity. Even though many efforts for optical satellite linkage have been started since the realization of the first laser in 1960, none of them were successfully technically completed (James E. Freidell "Commercial Opportunities, Versus Government Programs, will likely drive the Future of Laser Communications" in "Free Space Laser Communications" VIII, G. Stephen Mecherle, ed., Proc. SPIE 2699, pp. 2 to 9, 1996).
Up to now, all important space agencies have been operating with demonstration projects for laser links, which in general were intended to research all aspects of communications with satellites in low earth orbits (LEO), in geostationary positions (GEO) and for the linkage with ground stations and with aircraft. For example, the US Space Agency NASA has ordered an impressive number of projects since the sixties, none of which, however, did reach an operational status because of technical difficulties. At present, the most important endeavor is the demonstration system for laser communications (LCDS), which represents a practical feasibility study for the optical information transmission between LEO, GEO, ground and aircraft terminals (D. L. Begley et al., "Proposed Near-Term 1 Gbps Space Laser Demonstration System", in "Free Space Laser Communications" VIII, G. Stephen Mecherle, ed., Proc. SPIE 2699, pp. 24 to 37, 1996).
LCDS was intended to prove the advantages of the laser link over RF technology, namely in relation to dimensions, weight, output and costs at very high bit rates of up to 1 Gbit/s. In accordance with these ideas, the weight of the laser terminal was to be limited to less than 40 kg, with a power requirement of less than 100 W. Monolithic laser diode arrays with wavelengths in the range between 810 and 860 nm and an optical output up to approximately 1.2 W are used in the optical transmitter, and also take care of pointing, acquiring and tracking the partner satellite. The optical reception includes a CCD (charged couple device) array and a quadrant avalanche photodiode detector (APD) for pointing, acquiring and tracking, and a further APD detector for receiving the broadband information signals. Nonreturn-to-zero (NRZ) signals are used for the communication channels in accordance with the principle of laser intensity modulation and direct photodiode detection (IM-DD) (M. E. Fritz et al., "Photonics Space Experiment On-Orbit Results" in "Photonics for Space Environment" IV, Edward W. Taylor, ed., Proc. SPIE 2811, pp. 106 to 114, 1996).
In accordance with the above cited US endeavor, it is disadvantageous that the possibilities of modern laser technology have not been fully exhausted by far, because this concept is based on classical diode laser technology and does not take into consideration either the neodymium laser sources or the more efficient method of coherent communications.
The Japanese ETS VI satellite which, although it missed the planned geostationary orbit, went into a large elliptical orbit, carried a 22 kg laser terminal using a diode laser (wavelength 830 nm) for the downlink at 1.024 Mbit/s, in which a telescope with a diameter of 7.5 cm, a CCD sensor and a quadrant detector have been installed, and which uses an avalanche photodiode for optical reception (K. Akari et al. "Performance Evaluation of Laser Communication Equipment On-Board the ETS-VI Satellite", in "Free Space Laser Communications" VIII, G. Stephen Mecherle, ed., Proc. SPIE 2699, pp. 52 to 59,1996).
Transmission is based on the classic principle of IM-DD. The ground station in Tokyo uses a 10 W argon-ion laser of 514.5 nm and a modulation bit rate in NRZ format of 1.024 Mbits/s.
A further satellite (OICETS), planned for 1998, is intended to carry an improved laser terminal being completed at this time. The idea is to provide a connecting link between the Japanese satellite and the European optical SILEX experiment with the planned satellite ARTEMIS (geostationary) and SPOT4 (low orbit). The terminal is intended to include two redundant diode lasers of 850 nm and 100 mW average optical modulating output, a bit rate of 49 Mbit/s, in NRZ format, and an APD receiver, and to employ the method of intensity-modulation-direct detection. The optical portion of the terminal is intended to weigh 100 kg, the associated electrical portion 40 kg, the required optical output 480 W during the acquisition phase (0.3 s), and 300 W during the communication phase. The goal of the experiments is to test the practical feasibility of the optical inter-satellite link (ISL) for pointing, acquiring and tracking of satellites as well as for data communication.
The fact that the Japanese endeavor, although it represents only a problem definition if it were to be carried out, merely utilizes only classical optical and opto-electronic components and modulating methods, is disadvantageous in this connection, so that therefore any innovative step is lacking. For this reason the transmission capacity is limited and it should be difficult to demonstrate the advantages of optical communications with the photonic hardware employed. The plus of the experiment would be the fact that the optical terminal is already in orbit and first measurements have been taken.
The already previously mentioned SILEX experiment is a further project of the European Space Agency (ESA), whose goal is the implementation and testing of a laser communications transmission system for satellites. The implementation of this project is expected to occur by the end of this century (Toni Tolker Nielsen, "Pointing, Acquisition and Tracking System for Free Space Laser Communication System" in Proc. SPIE 2381, pp. 194 to 205, 1995).
The optical SILEX terminal uses diode lasers of a wavelength between 800 and 800 nm and with a bit rate of 2.048, or respectively 50 Mbit/s for the forward or respectively return connection. It is intended to transmit over up to approximately 45,000 km and have an electrical power requirement between 160 to 180 W. The relatively large dimension and the large weight of this terminal have led the ESA to order the construction of a small optical terminal (SOUT), which is intended to find space in an LEO satellite in order to take over the communication with a GEO satellite. Various bit rates are being tested, as a rule they depend on the emitted output of the diode lasers, which can lie between 50 mW and 1 W, if it is possible to assure a sufficiently long service life of the new high output laser diodes. When using such diodes, the bit rate in the NRZ communications format should be some Mbit/s. In any case, there is the expectation for SOUT to have a weight below 25 kg at 40 W power usage, which would represent a clear improvement over the first SILEX terminal version. But a further development with the designation SOTT is also already being tested, which is intended as a link between geostationary satellites at bit rates of several Mbit/s.
Technological developments in the field of the optical components and the laser sources during the last year result in problem definitions and therefore lead to the expectation of a continuous improvement of the properties of laser terminals. The appearance of monomode diode lasers of high optical output and of laser arrays permits the design of compact terminals with telescope diameters of approximately one inch. Thanks to diffractive optical devices and modern optical design, the total number of optical components can be greatly reduced, which results in a clear improvement in respect to weight, volume and dependability (Manfred Wittig "Optical Space Communications: How to Realize the Second Generation of Small Optical Terminals", IOS Press, Space Communications, pp. 55 to 89, 1994).
With this proposed terminal, diode lasers are employed for communication and for pointing, acquiring and tracking (PAT) the partner satellite. The PAT sub-system is a central component of an optical satellite link. The complexity of PAT is based on the requirement of locating a satellite from another one over distances of thousands of kilometers and at a beam divergence of a few millimeters. Mechanical vibrations of the transmitter and noise because of background radiation increase the bit error rate (BER) and therefore decrease accuracy and dependability. The new developments foresee a two-step pointing mechanism in order to achieve a large dynamic range. An additional diode laser beam of several Watt output can take over the job of detection, if it is guided in the form of a guide ray through a small telescope of approximately 5 mm diameter, which is mounted next to the main telescope. The tracking mechanism must have high immunity against interference by the background radiation, for example sunlight. The challenge of realizing a dependable functioning optomechanical structure for PAT lies in developing very low-vibration optical-mechanical devices, which at the same time are very light and small and require extremely low electrical output for controlling the optical devices and for electronic signal processing.
As the above cited problem definitions and remarks show, the next generation of communication satellites requires broadband, digital signal processing on board, in contrast to the present prior art, which in principle is a simple relay technology. Furthermore, solutions based on refined optical devices are lacking. Also, no solutions employing small and handy laser sources of high radiation output, such as diode-pumped neodymium-YAG lasers, are known. The latest work in this field by ESA attempts to treat outstanding questions by means of modern technology. However, in accordance with the sources in the prior art as cited above, it is unmistakable that no known problem definitions for developing laser communication links between LEO or GEO satellites and ground stations disclose a total solution which would be in a position to end in a product representing a real alternative in respect to the dominating RF technology in this field (Stephen G. Lambert and William L. Casey "Laser Communications in Space", Artech House 1995, pp. 279 to 294).